Facies analysis and depositional evolution of Lower–Middle Ordovician carbonates in the Shuntuoguole Low Uplift of Tarim Basin (NW China)

As a result of a deep burial depth (> 6.5 km), the depositional history of the Early to Middle Ordovician carbonate platform in the Shuntuoguole Low Uplift (Tarim Basin, NW China) has been difficult to unravel, leading to problems in understanding the controls of the facies distribution, particularly in terms of petroleum exploration. Integrated with geophysical surveys, detailed sedimentological observations document the platform evolution and show that a carbonate ramp system developed in the Shuntuoguole Low Uplift during the Early-Middle Ordovician, with twelve lithofacies types arranged into two facies associations, including peritidal to semi-restricted subtidal and open-marine subtidal facies. Based on the tempo-spatial distribution of lithofacies, seven third-order depositional sequences are identified, each composed of a transgressive lower package and a regressive upper part. Their sequence boundaries are commonly characterised by a transitional zone with no obvious subaerial exposure features. The correlation of relative sea-level (or accommodation space) changes derived from the vertical stacking pattern of facies with the global sea-level curve of Haq and Schutter (2008) and sea-level oscillations of coeval successions worldwide suggests that eustatic sea-level fluctuations were the overriding factor controlling the development and evolution of this carbonate ramp during the Early-Middle Ordovician. Meanwhile, regional or local tectonic subsidence and environmental factors (e.g. palaeoclimate and prevailing wind) also exerted an influence on the depositional processes of the Shuntuoguole Low Uplift over this time interval.